1. Technical Field
The present invention relates to increasing the brightness of III-Nitride light emitting diodes.
2. Description of Related Art
Light emitting diodes (xe2x80x9cLEDsxe2x80x9d) are a highly durable solid state source of light capable of achieving high brightness and having numerous applications including displays, illuminators, indicators, printers, and optical disk readers among others. Direct bandgap semiconductors are the materials of choice for fabrication of LEDs, which generate light from electricity. One important class of light emitting systems are based upon compound alloys of Group III atoms (particularly In, Ga, Al) and nitrogen N, typically abbreviated as xe2x80x9cIII-Nitrides.xe2x80x9d One family of III-Nitride compounds has the general composition (InxGal-x)yAll-yN where 0xe2x89xa6(x,y)xe2x89xa61. III-Nitrides are capable of emitting light that spans a large portion of the visible and near-ultraviolet electromagnetic spectrum including ultraviolet, blue, green, yellow and red wavelengths. Improving the brightness and other optical properties of LEDs is an important technological goal.
A portion of a typical prior art LED structure is depicted in FIG. 1. Other components of LEDs as known in the art (electrodes, window materials, etc.) are omitted for clarity.
An LED typically has one or more layers epitaxially deposited on a surface of a substrate prior to the formation of the light emitting active region. These epitaxial layers form a xe2x80x9cbase layerxe2x80x9d that can have n-type conductivity. FIG. 1 depicts an example of a base layer having a GaN layer beneath an n-type GaN layer.
The light emitting active region in which radiative recombination of electrons and holes occurs is formed on top of the base layer, typically in the form of at least one quantum well although single and double heterostructures and homojunctions can also be used. Above the active region lie p-type conductive injection and confinement regions. Positive and negative contacts (omitted from FIG. 1) are also provided.
There remains a need for LEDs with improved optical performance including higher LED brightness and higher quantum efficiency.
The present invention relates to structures for light emitting devices, particularly LEDs employing a III-Nitride light emitting active region deposited on an n-type conductive base layer. The substrate upon which the base layer is grown is cut intentionally misaligned from a main crystal plane. In addition to intentional substrate misalignment, base layers are employed that are thicker than 3.5 xcexcm. In some embodiments of the present invention, the presence of a thick base layer provides sufficient mechanical support for the device such that the substrate can be removed entirely from the light emitting system, further increasing the performance of the device.
Examples are provided for the illustrative case of thick base layers deposited on a sapphire substrate misaligned from the c-axis. Misalignment angles are in the range from 0.05xc2x0 to approximately 0.50. The present invention also employs base layers thicker than 3.5 xcexcm, preferably in the range of 7 xcexcm-10 xcexcm. The combination of base layers greater than 3.5 xcexcm, grown on the misaligned substrates leads to surprisingly improved light emission.